Package-on-package (PoP) structures have been developed for applications such as cellular telephones in which circuit board space must be conserved. The top package is typically a memory package whereas the bottom package is generally a processor package. Package-on-package technology has proven to be quite popular as compared to other approaches such a stacked-die circuit. For example, a manufacturer can readily substitute different memory packages in a PoP circuit, which thus lowers costs as opposed to being tied to a particular memory. Moreover, the top and bottom packages may be tested independently. In contrast, a bad die in a stacked-die design requires rejection of the remaining good die.
Although the packaging of integrated circuits using a PoP architecture is thus quite popular, challenges remain in this packaging process. For example, one challenge involves accommodating a decreased interconnect pitch. As technology advances, a circuit needs ever more signal pathways to achieve greater speed and versatility. To accommodate more signal pathways in the same package size, a PoP structure needs finer interconnect pitch. But the bottom package has a certain standoff height with regard to the top package (a required height or separation between the top package substrate's bottom-package-facing surface and the bottom package substrate's top-package-facing surface). In a conventional low-density PoP circuit with relatively large pitch, the standoff height of the die is readily accommodated because the solder balls can be made correspondingly large such that they can reach the bottom package substrate by spanning across the standoff height separation between the top and bottom package substrates. But as the pitch is decreased in modern designs, the solder ball size must also be decreased. This means there is a stand-off issue between the upper and lower packages for high-density (low-pitch) designs because the solder balls on the upper package cannot extend to the lower package due to the standoff height for the die on the lower package.
This standoff height issue will now be explained further using the terms “first package” and “second package” as compared to the use of bottom package and top package, respectively. The art-recognized definitions of “top package” and “bottom package” as known to those of ordinary skill in the PoP arts are not tied to any coordinate system such that a bottom package does not become a top package merely because a PoP is flipped upside down. But to avoid any ambiguity, the following discussion will designate the bottom package as a “first package” and the top package as a “second package.” With these concepts in mind, the standoff height issue may be better understood with reference to a conventional PoP 100 of FIG. 1. A second package substrate 105 rests on a first package die 110 to minimize the standoff issue. Since first package die 110 is not encapsulated in mold compound, the first package may be denoted as a bare-die first package. In a fine-pitch design such as less than or equal to 0.4 mm pitch, a plurality of second package solder balls 115 projecting from a first-package-facing surface of second package substrate 105 must be made sufficiently small to accommodate such a fine pitch. Thus, second package solder balls 115 cannot contact a first package substrate 120 due to the standoff issue. Second package balls 115 are thus not providing electrical continuity between the first and second packages.
To achieve electrical continuity despite the standoff issue, a set of first package solder balls (not illustrated) may be placed on a set of pads 125 on the second-package-facing surface of first package substrate 120. Second package solder balls 115 may then reach to these first package solder balls, thus solving the stand-off problem. But it is problematic to have unguided ball-to-ball coupling in that the round shapes inherently lead to misalignment. Thus, it is often necessary to flatten or coin the first package solder balls such that they have a flat surface instead of a rounded surface. Such processing steps add cost and complication to modern high-density designs. To reduce the standoff issue further, first package die 110 can itself be ground down to achieve a lower standoff height. For example, first package 110 may need to be ground to approximately 0.06 mm to achieve a standoff height of approximately 0.13 mm. In this fashion, second package solder balls 115 having a pitch of 4 mm or less can reach to first package substrate 120. But that leaves the ground-down die very fragile and prone to breaking. Moreover, the grinding of the die is expensive.
To avoid the need for solder ball flattening or coining, it is also conventional to use an over-molded first package in which the first package solder balls and first package die 110 on first package substrate 120 are encapsulated with an suitable encapsulating agent such an epoxy mold compound (EMC) or resin 130 as shown in FIG. 2 for an over-mold PoP structure 200. Vias are then laser ablated or mechanically cut through EMC 130 to expose the first package solder balls on first package substrate 120. These vias help guide the second package solder balls into an appropriate ball-to-ball alignment with the first package solder balls when the second package is stacked onto the first package. PoP structure 200 may then be reflowed to form uniform solder structures 205 from the aligned solder balls. But the laser ablation or mechanical cutting step increases manufacturing cost and complexity for over-molded PoPs.
Accordingly, there is a need in the art for improved high-density PoP architectures.